Nuclear electricity-generating power plants of practically all types from time to time must dispose of radioactive waste water which can be derived from secondary or tertiary coolant cycles, from water in contact with contaminated materials or zones, or from the steam-generating system.
In general the radioactive waste water which must be disposed of often contains solids, especially boric acid, which are in dissolved form.
A conventional disposal technique is to store the radioactive waste water for a period sufficient to allow decay of some of the radioactive substances therein and then subject the stored water (with reduced radioactive level) to waste-water processing by any of a number of techniques including chemical precipitation or biological treatment.
A disadvantage of this approach is the need to store relatively large quantities of water for long periods of time.
It has also been proposed to concentrate the waste water and thereby reduce the volume of this substance which must be handled. In this conventional process, the waste water is concentrated by evaporation and the evaporation is carried out until the solids concentration in the water is at a level less than that which would represent a saturated solution at room temperature. The water is then stored for decay of radiation, e.g. for one half to three quarters of a year and then packaged, e.g. by incorporation in a solid mass, for permanent disposal and transportation.
The permanent disposal may involve mixing the concentrated water with cement, (e.g. hydraulic cement) or incorporating the water in a hardenable bitumen or in a synthetic resin mass.
In all cases the hardened material constitutes a leach-resistant body which can be sealed in a container, canister or drum with or without significant radiation-shielding capacity, the resulting package being given subterranean storage or being otherwise disposed of by techniques conventional in this art.
The incorporation of the radioactive waste, whose activity has been reduced by long-term storage, in a hardenable mass prevents contamination of the environment in a particularly effective manner and the concentration step reduces significantly the volume of the material which must be handled in this manner.
However, the degree of concentration is limited in the prior art process by the need to prevent the concentration of solids, during evaporation, from reaching the saturation concentration at room temperature, thereby ensuring that no solids will precipitate from the water and deposit in the system.
The storage vessels which are commonly used for the radioactive decay process may have volumes of about 60 m.sup.3 and consequently, the cost of a storage facility for the interim storage of the waste water can be considerable and the operating cost of the power plant correspondingly high.